Preheat oven to 350 F. Butter or spray a bundt pan well, making sure you grease all the creases. Bring the stout and butter to a simmer in heavy large saucepan over medium heat. Add cocoa powder and whisk until mixture is smooth. Cool slightly. Whisk the flour, sugar, baking soda, and salt together in large bowl.

Using an electric mixer, beat eggs and sour cream in another large bowl to blend. Add stout-chocolate mixture to egg mixture and beat just to combine. Add flour mixture and beat briefly on slow. Using rubber spatula, fold batter until completely combined. Pour batter into the greased bundt pan, and bake until it passes a cake test, about 35 minutes.

Transfer cake to rack and cool completely in the pan. Turn cake out onto the rack and serve either with ganache, or your favorite topping.

For the ganache (optional):
Bring cream to simmer in small saucepan. Remove from heat; add chocolate and whisk until smooth. Pour over cake, or reserve for individual servings. (Can be made one day ahead: Chill, rewarm before using.)

To read Jennifer Ward Barber's post about making this cake for St. Patrick's Day, click here.